This invention relates generally to telecommunications systems, and in particular to digital communications systems employing higher order linear modulation.
Digital communications systems for the transmission of information, or data, are commonplace. An information-bearing signal generally consists of a stream of data symbols, each one being selected from a set of possible discrete values or levels, e.g., +1 or xe2x88x921, and can be viewed as a xe2x80x9cbasebandxe2x80x9d signal because it has a frequency spectrum ranging from DC to approximately the maximum data rate in symbols per second. To more efficiently transmit the information through a medium, or channel, a transmitter may xe2x80x9cmodulatexe2x80x9d the data onto a xe2x80x9ccarrierxe2x80x9d sinusoid. A local reference generator at the transmitter produces the carrier sinusoid of frequency fc, which is usually selected to be much higher than the maximum baseband frequency. Data modulation is simply the modulation of the carrier""s frequency, phase and/or magnitude, according to the data bit values. As a result of this modulation, the information-bearing signal is shifted from baseband up to the carrier frequency fc, which is chosen to avoid interference with other transmitted signals.
One of the first functions of a receiver, therefore, is to shift, or demodulate, the information-bearing component of a received signal back down to baseband by multiplying it with a local reference of frequency fc, This frequency-shifting function, referred to as xe2x80x9ccarrier recoveryxe2x80x9d or xe2x80x9cphase trackingxe2x80x9d, must be very accurate; otherwise, the receiver may make errors during detection when it determines the data bit values represented by received symbols. Unfortunately, an accurate down-conversion is difficult due to phase variations introduced into the received signal by two separate processes. First, in wireless channels, the transmitted signal may be is Doppler shifted as it passes through the channel due to relative motion between the transmitter and receiver. Second, the local reference at the transmitter may be out of phase with the local reference at the receiver. In fact, the phase error may even be time varying.
Carrier recovery is commonly performed at the carrier frequency, wherein a carrier phase estimate derived from the demodulated signal is used to dynamically adjust the phase of the receiver""s local reference. However, when the carrier is complex-modulated, i.e., the data bit value is encoded into the phase of the transmitted data symbol, carrier recovery at radio frequencies is particularly difficult. Most conventional systems employ either a non-linear operation, such as a Costas loop or a squaring loop, or a decision-directed operation to track the carrier. In the first case, the received signal is passed through a non-linear function to generate power at the carrier frequency onto which a phase-locked loop (PLL) can lock and then track the carrier. The complexity of a non-linear operation increases with the order of modulation, however, and is thus impractical for use with higher-order modulation schemes. Differential encoding at the transmitter, along with differential decoding at the receiver, is also required because of a fixed phase ambiguity between the estimate produced by the non-linear operation and the received signal. For high-order modulation schemes, decision-directed operation can be more easily implemented than the techniques based on non-linear operations, but it works poorly when a received signal has a low signal-to-noise ratio (SNR), i.e., a bit error rate greater than 10xe2x88x922. A high bit error rate can also be caused by signal distortions, for example , time dispersion. Since the decision-directed techniques rely on correct decisions, significant levels of these distortions are not tolerable.
Therefore, what is needed is a method for tracking the carrier phase of a received signal in a telecommunication system employing higher-order complex modulation. Such a method should not require modification of the information-bearing signal and should be capable of performing in less than optimal channel conditions.
A method and system for tracking the carrier phase of a received signal includes the addition of a pilot signal (also referred to herein as a pilot tone) to the information-bearing component of the signal prior to modulation and transmission. The received signal is demodulated to produce a complex data signal from which the pilot tone is detected to thereby provide an estimate of the phase error in the complex data signal. The elimination of the phase error from the complex data signal is accomplished through a corrective phase shift that is equal and opposite to the estimated phase error. A sample timing estimate may be obtained from the pilot tone in a similar manner. The phase and frequency of the pilot tone are selected such that the pilot tone does not interfere with the information-bearing component of the transmitted signal. Therefore, other than the simple addition of the pilot tone, no modification of the information-bearing component is required. Furthermore, the method can be readily implemented in higher-order modulation schemes and, because pilot tone detection is insensitive to the exact magnitude of the pilot tone, the method is also robust.